During the past fifty years, tremendous progress has been made in electronic technologies. Large vacuum-tube-based electronic systems were replaced with smaller, more reliable transistor-based electronics, in turn replaced with integrated-circuit technologies with ever increasing component densities and switching speeds. Currently, efforts are being made to push the standard photolithographic semiconductor-fabrication process to produce ever decreasing components sizes. One approach to decreasing component sizes is to decrease the wavelength of the light used to expose photoresist in photomasking steps. Retooling current semiconductor-fabrication facilities to employ shorter wavelength, soft x-rays is projected to involve staggering capital expenditures. Moreover, various physical limits of component sizes may be soon approached. For these reasons, designers and manufacturers of electronics systems, and, in particular, manufacturers and designers of computer processors and memories, are currently seeking non-photolithographic technologies for producing denser electronic circuitry.
A new approach to circuit fabrication involves molecular electronics. Nanoscale molecular wires (“nanowires”) can now be fabricated into programmable nanoscale crossbars and other structures, with molecular junctions at nanowire interconnections, comprising as few as 40 molecules, providing resistor-like, diode-like, and transistor-like nanoscale components. Efforts are currently underway to fabricate high-density logic circuits, memory elements, programmable gate arrays, and processors using these new nanoscale technologies. However, at nanoscale dimensions, precise fabrication of molecular-junction diode and transistors presents many challenges. Designers and manufacturers of nanoscale electronic circuits, including logic circuits and memories, have therefore recognized the need for basic, reliable molecular logic gates despite current difficulties in reliably fabricating nanoscale diode and transistor components.